(a) Field of the Invention
The present invention relates to a hydraulic control system of an automatic transmission for vehicles. More particularly, the present invention relates to a hydraulic control system of an automatic transmission capable of more precise control of the operating hydraulic pressure supplied to frictional elements.
(b) Description of the Related Art
In an automatic transmission, a TCU (transmission control unit) controls a plurality of solenoid valves of a hydraulic control system based on various conditions, such as vehicle speed and opening of a throttle valve, so as to control the operating pressure supplied to a plurality of frictional elements (e.g. clutch and brake) that are applied to a gear train and selectively operate at a target shift ratio, and thereby automatically achieving a specific shift ratio. In such an automatic transmission, a clutch consisting of frictional elements must be supplied with a maximum hydraulic pressure corresponding to a torque load for a corresponding clutch under which slip can be prevented at a maximum input torque for each shift ratio. A precise hydraulic pressure control should be performed within a suitable range in order to achieve a smooth shift feel while performing an operating control or releasing control of a clutch.
In addition, the hydraulic pressure control range during shifting may be lower than a maximum pressure designed considering stall torque ratio of the torque converter. The minimum pressure may be 0 or a specific value for pre-fill characteristics. A pre-fill value of the minimum pressure may be realized by a pressure control valve or a regulating ball connected to an exhaust port of a hydraulic pressure control valve.
Thus, since the hydraulic pressure control range during shifting is relatively shorter compared to a range of the maximum pressure and the minimum pressure, the control range of the control pressure of a solenoid valve is limited. Also, the sensitivity of the clutch control pressure with respect to the change of a solenoid valve control pressure is reduced. Therefore, a difference in shift feel becomes great depending on the deviation of the control pressure of a solenoid valve.
In particular, in the case where there is a great difference in torque load between shift ratios in a multi-stage transmission, such as a six-speed transmission, there can be a problem in that the hydraulic pressure control range during shifting is very restrictively set as 20 to 30% of a maximum pressure. In order to solve such a problem, in the conventional control of operating pressure supplied to a frictional element, if a shift signal is generated by a TCU, a solenoid valve controls a solenoid supply pressure and supplies the controlled pressure to a pressure control valve. For such a control, the force from the maximum control pressure of the solenoid valve is set to be greater than the sum of the force of a maximum control pressure of a clutch and a spring elastic force.
However, in a case of controlling hydraulic pressure supplied to a frictional element, since an operating hydraulic pressure of the frictional element is controlled by a pressure control valve, having a narrow control pressure range, there can be a problem in that a precise hydraulic pressure control is difficult. In addition, since the solenoid valve control pressure directly controls the pressure control valve, a change of operating hydraulic pressure of a frictional element depending on control pressure deviation is relatively great. Accordingly, precise hydraulic pressure control is further complicated, and may not be possible. Also, in the case where there is a great difference in torque load between shift ratios in a multi-stage transmission, such as a six-speed transmission, the control range of a solenoid valve becomes narrow, so that a precise hydraulic pressure control is rendered even more difficult.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.